The study of intergranular corrosion in aircraft aluminium alloys using X-ray tomography

Atmospheric corrosion is one of the leading causes of structural damage to aircraft. Of particular importance is pitting and intergranular corrosion, which can develop into fatigue cracks, stress corrosion cracks, or exfoliation. Therefore it is of interest to the Australian Defence Force (ADF) to understand how corrosion ensues in susceptible aircraft aluminium alloys, such as AA2024-T351 and 7050-T7451. However, there are many difficulties in measuring the extent of intergranular corrosion, since it is predominantly hidden below the surface. Traditionally, cross-sectioning has been used to view and measure the depth of attack. In the present work, 2 mm diameter pin specimens were contaminated with a droplet of 3.5% NaCl and exposed to constant humidity that resulted in intergranular corrosion. X-ray computed tomography was then used to non-destructively assess the depth and volume of corrosion both as a function of time in 97% relative humidity, and as a function of relative humidity after 168 h exposure. Both corrosion depth and volume increased with time, but there was evidence for a limiting depth in AA2024. Depth and volume also increased with relative humidity of the environment, for which the time-of-wetness and oxygen concentration of the droplets were considered the important factors in driving the corrosion process.

Research highlights
► IGC is stochastic, where initiation is statistical and growth kinetics was somewhat predictable.
► Dissolved oxygen concentration was more important than the concentration of salt in the droplet.
► A limiting depth occurred for AA2024, whereas no limiting depth occurs for AA7050 after 168 h exposure.
► A limiting depth may be controlled by the transport of dissolved oxygen down the corrosion fissure.
► A limiting IGC depth is dependent on the overpotential of the SDZ (adjacent to the grain boundary).